Discovery and geological significance of methane-rich fluid inclusions of Chaihulanzi gold deposit in the northern margin of the North China Craton

The Chaihulanzi gold deposit is located in the western section of the Chifeng-Chaoyang gold deposit on the northern margin of the NCC. It is a typical quartz vein type gold deposit, and is also the first gold deposit with a large number of single-phase methane inclusions found in the Chifeng-Chaoyang area. The ore body occurs in the fault, cutting across the quartz in the graphite-bearing sericite schist of the lower member of the Dayingzi Formation in the Archean Jianping Group. Gold occurs mainly in the altered graphite-bearing sericite schist and quartz veins. According to the results of field observation and inclusion temperature measurement, the fluid mineralization process of the Chaihulanzi gold deposit can be divided into the main mineralization period and the late mineralization stage, which are quartz vein and altered rock type ore and ore free quartz and calcite vein combination respectively. The results of fluid inclusion petrography and temperature measurement show that the inclusions in the main ore-forming stage are mainly water-salt two-phase inclusions and single-phase pure methane inclusions. The homogenization temperature of water-salt inclusions ranges from 135℃ to 377℃ with an average of 256℃, and the salinity ranges from 3.06% to 14.04%. The homogenization temperature of methane inclusions ranged from -107.6℃ to -79℃, with an average of -88.8℃. Laser Raman spectroscopy analysis showed that the gas phase composition of water-salt inclusions was mainly water, containing a small amount of CO2, and a very small amount of inclusions showed obvious peaks of methane and water. In the methane inclusions, CH4 is the only component, and there is no water peak. In a few inclusions, the characteristic peaks of CO2 and CH4 appear simultaneously. The fluid inclusion population also shows that the inclusions in the main ore-forming period are mainly aqueous solution inclusions, and the methane-rich inclusions are obviously developed. Meanwhile, the inclusions contain more CO2 and N2, which is consistent with the results of measurement temperature and laser Raman spectroscopy analysis. In the late mineralization period, only water-salt inclusions developed in the rocks, the homogenization temperature ranged from 175℃ to 313℃, with an average of 250℃, and the salinity ranged from 1.57% to 9.47%. The H-O-S isotope analysis shows that the ore-forming fluid of the Chaihulanzi gold deposit is mainly from magmatic water, the ore-forming materials are mainly from deep magma, and the methane in ore-forming fluid comes from the metasomatism reaction between deep source flow body and graphitic surrounding rock. According to the isovolumic intersection diagram of the symbiotic combination of water-salt-methane inclusions in the main ore-forming period, the capture temperature of ore-forming fluid is 264~340℃, the average is 302℃, the pressure is 102~154 MPa, the average is 128 MPa, pH-logfO2 diagram shows that the ore-forming fluid pH=4.25~5.05, logfO2=-34.85~-34.54, the gold complex is Au(HS)2-, and the solubility of gold is 10×10-9~46.6×10-9. Based on the above research, the low gold content deep ore-forming fluids reacts with graphite-bearing sericite schist of the Dayingzi Formation, the reaction equation is 2C+2H2O=CH4+CO2. The resulting methane has a low solubility under the temperature and pressure conditions in the main ore-forming stage, resulting in the boiling reaction of the gold-bearing fluid, which is the main mechanism of the Chaihulanzi gold precipitation and mineralization, and CO2 appeared in the form of calcite in the late mineralization stage. © 2022, Science Press. All right reserved.